1. Field of the Invention
The present invention relates in general to a method and apparatus to set and apply tension to casing or completion tubing in a wellbore, and in particular to a tubing hanger having an inner member and an outer member, and a running tool that sets the outer member, draws tension on the tubing by pulling the inner hanger, and then maintains the tension by locking the inner hanger into the outer hanger.
2. Brief Description of Related Art
Some wells, such as gas injection storage wells, have completion strings comprising tubing. The completion strings experience thermal expansion due to temperature variations when, for example, gas is injected into a storage well or withdrawn from a storage well. To compensate for the thermal expansion, the tubing may be placed under tension. With sufficient tension, the thermal expansion merely relaxes some of the tension. The travel distance associated with thermal expansion is less than the distance the tubing was stretched during the tensioning. Thus, even when the tubing expands due to increased temperatures, the tubing does not buckle within the wellbore.
Tensioning devices currently used on gas storage wells use retractable load shoulder arrangements which are often based on blow-out preventer designs. These designs require through-wall penetrations in the main pressure-containing housing, thus creating potential leak paths. This type of design also results in increased cost of the wellhead as the main housing material has to increase in diameter to accommodate the actuating mechanisms, which results in increased manufacturing costs and in addition, costs for the retractable load shoulder mechanism.
Modern well practice is to run various downhole safety valves and gauges through the wellbore. The existing retractable load shoulder type tensioning arrangement causes interference problems with the associated control lines descending below the tubing hanger.
Whilst the retractable load shoulder arrangement is relatively simple from a mechanical standpoint, it leads to the use of elastomeric materials to provide the main well bore seals. It is widely known that elastomeric materials degrade over time and given that gas storage facilities are usually planned to have long service lives (up to forty years), this seal degradation causes problems in later years.